Pulse stuffing, as a general concept, is well described in many prior art references, one example being a Johannes, et al., U.S. Pat. No. 3,461,245, assigned to Bell Telephone Laboratories.
In the prior art, the loopback of signals, for DS1 data format signals only, has been accomplished by the present inventor using a technique somewhat similar to the present. A stuff bit code was intentionally violated on a periodic basis for the DS1 transmission lines. Since the overhead data utilized for a DS1 transmission channel can be accepted without change by either a DS2 or DS3 high speed transmission line, the entire data channel was merely transmitted across the DS2 or DS3 transmission line. At the destination, the violation of the stuff code was detected and switches were set to provide loopback of the signal to the source. A stuff code detector used a voting circuit and relied on two out of three bits being correct and thus, the violation of the third bit did not normally affect correct transmission of stuffing data. The loopback connection was set only after a predetermined number of consecutive violations had been detected and was maintained only as long as these violations continued to be detected.
The concept, as described above, is not immediately applicable to the transmission of DS1C data since the overhead bits are stripped from the DS1C signals before the signal is divided into two DS1 signals. Thus, the present invention does not insert the violation of the stuff code until after all of the data is in the DS2 format wherein the DS1C signal comprises two subframes of a frame in the DS2 format. Since each subframe has its own set of overhead bits as defined in Technical Advisory No. 50 from American Telephone and Telegraph Company, the loopback detection and switching equipment at the destination end can set whatever number of switches is appropriate.
In view of the above, it is an object of the present invention to provide a solution to the problem of "looping back" signals from a remote destination where the signal being looped back does not retain its original overhead bits in the transmission medium.